I am about to tile my new bathroom with a wooden sub floor structure and want to make sure that my tiles or grout arenâ€™t going to crack due to movement. Does the following sound like a good idea as a sub floor before I lay my tiles? On top of the wooden floor boards Iâ€™ll put a 12 mm backing board and on top of that Iâ€™ll put my tiles â€“ would this be enough to prevent movement or should I put a layer of plywood first then backing board and then my tiles?
If the backing board is more flexible than the plywood then you need the plywood but 18mm would be better. Timber floors move a great deal and backing boards are there for greater adhesion rather than rigidity.